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Elucidating the Role of the ERM Family Adaptors, Ezrin, Radixin and Moesin, in Breast Cancer Cell Drug Resistance and Metastasis
Introduction:
- Therapeutic strategies for breast cancer treatment
- Overview of breast cancer
Breast cancer is the most common cancer among Canadian women, accounting for an average of 72 new diagnoses each day nationwide1. Breast cancer originates from cells in the breast that begin to grow uncontrollably and deviate from the normal process of cell division. The majority of breast cancers arise from cells that line the mammary ducts which carry milk from the mammary glands to the nipple. This type of breast cancer is called ductal carcinoma. In contrast, lobular carcinoma originates from the cells of the lobules, which are the milk-producing glands of the breast. Both of these breast cancers possess the ability to progress to an invasive metastatic stage in which treatment is often unsuccessful.
Staging is used to describe a cancer based upon the amount of cancerous tissue present and where in the body the cancer was first diagnosed. The most common staging system for breast cancer is the TNM system which consists of five stages. In situ breast cancer, which describes cancer cells that are non-invasive, is labelled as “stage 0” while metastatic breast cancer that has spread to other parts of the body, such as the bone, liver, lungs or brain, is labelled as “stage 4” and correlates to the most devastating prognosis.
As with many diseases, prognosis and survival for breast cancer patients depends on a wide variety of factors including age and menopausal status at diagnosis, medical history, stage of the cancer and the treatment strategies employed. Treatment options often revolve around the expression status of the estrogen receptor (ER), human epidermal growth factor type 2 receptor (HER2) and progesterone receptor (PR). When breast cancer has progressed to a metastatic stage, more aggressive treatment options such as combination chemotherapy are often required.
- Chemotherapy
The era of modern chemotherapy can be traced directly to the discovery in the 1940s of nitrogen mustard as an effective cancer treatment2. A chemotherapeutic approach to treating cancer is based upon the concept that tumours, due to their tendency to display an increased rate of cell division, are more susceptible to certain toxins than normal tissues. These chemotherapeutics typically interfere with processes important in cell proliferation, particularly DNA replication, to trigger programed cell death pathways and eradicate cancer progression. This strategy was founded upon observations of autopsies of soldiers who were exposed to sulphur mustard gas during the First World War and displayed profound lymphoid hypoplasia and myelosuppression. Originally proposed by Louis Goodman and Alfred Gilman, a novel treatment strategy for a patient with advanced non-Hodgkin’s lymphoma proceeded by injecting into the bloodstream the closely related compound, nitrogen mustard3. Although only a few weeks of remission were observed before further disease progression, the ground-breaking principle that certain toxins could be administered systemically to promote tumour regression was established.
Following the end of the Second World War, an additional innovative approach to cancer drug therapy emerged as the effects of folic acid on patients with acute lymphoblastic leukaemia (ALL) were explored4. By blocking the function of folate-requiring enzymes, the folate analogue methotrexate became the first drug to successfully induce remission in children with ALL5. Methotrexate was subsequently proved to have anti-tumour activity in a wide variety of epithelial malignancies, including breast and ovarian cancers.
These discoveries quickly progressed and in the late 1960s evolved into more specific treatment strategies for metastatic breast cancer when combination chemotherapy in the adjuvant setting emerged at the forefront of cancer research6. In 1973, a clinical trial utilized combination chemotherapy with cyclophosphamide, methotrexate, and 5-fluorouracil (CMF) to treat women with operable breast cancer7. Preliminary findings showed that patients within the adjuvant CMF group displayed significantly better overall survival than those who received only a mastectomy7. It was found that administering a combination of drugs, rather than a single agent, was a more effective treatment strategy against both metastatic cancer and for patients at high risk of relapse after primary surgical treatment4.
These foundational discoveries provided the basis of modern systemic chemotherapy, which are currently administered for treatment of a plethora of different cancers. Today, anthracyclines such as doxorubicin, are commonly used as chemotherapeutic agents to treat breast cancer. This drug prevents the DNA double helix from being relaxed out of its supercoiled formation by blocking the activity of the enzyme topoisomerase II. This mechanism arrests the cell replication process in tumour cells.
- Additional therapeutic approaches
Recent discoveries within the field of medical oncology have provided valuable insights into the heterogeneity of breast tumours, key oncogenic drivers and the role of the immune system in breast cancer8. These developments have allowed many novel therapeutic strategies to emerge. As immune escape is a well-known hallmark of cancer, researchers have developed immunotherapy treatments that work to reactivate the host immune system to attack and eradicate tumours9. The immune system normally uses regular “checkpoints” to inhibit unnecessary T-cell activation in order to prevent autoimmunity, however this system becomes dysregulated in cancer. As a result, drugs that target these “checkpoints” hold a lot of potential for cancer treatments. The cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) and programmed death protein (PD-1) are members of the same class of co-inhibitor receptors expressed on activated T cells. As negative regulators of T-cell function, these proteins works to attenuate further T-cell activation during antigen presentation, as well as regulate immune reactions downstream within the tumour microenviornment8. Antagonistic monoclonal antibodies have been developed to block both CTLA-4 and PD-1 activation, proving to be promising therapeutics for breast cancer, especially when used in combination8.
Inhibition of enzymes by means of small molecules, such as tyrosine kinase inhibitors (TKIs), has also been important in recent therapeutic developments. TKIs work to inhibit the initial autophosphorylation of the kinase domain upon ligand binding, effectively inhibiting downstream signals for survival and proliferation pathways10. Lapatinib is the first TKI developed to treat breast cancer that targets both HER2, a surface protein critical to cell signaling pathways and the epidermal growth factor receptor (EGFR)11.
- Overview of ERM proteins
- Structure of ERM proteins
The ezrin-radixin-moesin (ERM) family is a class of highly homologous proteins involved in linking the plasma membrane to the cortical actin cytoskeleton. Through evolution, this family has been greatly conserved, presenting more than 75% amino acid identity that are shared between ezrin, radixin and moesin12. Figure 1 illustrates that structurally, ERMs are characterized by the presence of an approximately 300 amino acid long plasma membrane-associated Four point one, ERM (FERM) domain at the amino terminus, also known as the N-terminal ERM association domain (N-ERMAD)13,14. Revealed through X-ray crystallography, the FERM domain consists of F1, F2 and F3 subdomains which fold to form a cloverleaf structure15. This FERM domain is directly followed by a central
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